The present invention relates to a method and apparatus for forming a film, particularly, a method and apparatus used for forming a semiconductor device and effective for forming, for example, a gate insulating film of a high reliability.
In recent years, attentions are paid to a silicon nitride film as a highly reliable gate insulating film of the next generation because the silicon nitride film has a dielectric constant higher than that of a silicon oxide film. To be more specific, in the case of using a silicon nitride film as a gate insulating film, a capacitance larger than that obtained in the case of using a silicon oxide film is obtained even if the gate insulating film is formed thicker. Therefore, the silicon nitride film is expected to overcome the difficulty accompanying the reduction in the thickness of the silicon oxide film, i.e., the difficulty in controlling the thickness of the gate insulating film. It should also be noted that, in the case of using silicon oxide for forming the gate insulating film, boron doped in a p.sup.+ polycrystalline silicon gate electrode tends to leak through the gate insulating film into the channel region. In the case of using silicon nitride for forming the gate insulating film, however, the gate insulating film can be formed to have higher film density and thicker, making it possible to prevent boron doped in the gate electrode from leaking through the gate insulating film into the channel region.
Several methods of forming a silicon nitride film are being studied, as summarized below:
(A) Thermal nitridation of a silicon substrate surface using NH.sub.3 : Required is a high temperature process of 1,000.degree. C. or more. The thickness of the nitride film is saturated at about 3 nm. PA1 (B) Deposition of a nitride film using a CVD method: Generally known are an SiH.sub.4 +NH.sub.3 system (for formation of an oxidation diffusion mask), an SiCl.sub.4 +HN.sub.3 system (for formation of a surface protective film), and an SiH.sub.2 Cl.sub.2 +NH.sub.3 system (for formation of an NMOS memory). The film-forming temperature is 700 to 950.degree. C. PA1 (C) Nitriding of a silicon substrate surface using plasma: Reported are a surface nitridation using a nitrogen plasma, a plasma anodic nitridation, and an impurity gas-added plasma nitridation (N.sub.2 +SF.sub.6). The process temperature falls within a range of between 700.degree. C. and 900.degree. C. in each of these processes. A nitride film having a thickness of scores of nanometers can be obtained. PA1 (D) Deposition of a nitride film using plasma: Many systems are being studied including an SiH.sub.4 +NH.sub.3 system, SiH.sub.4 +N.sub.2 system, Si.sub.2 F.sub.6 (or SiF.sub.4)+N.sub.2 system, and an SiH.sub.4 +NF.sub.3 system. Also, an electron cyclotron resonance plasma (ECR), etc. are being studied in addition to the radio frequency plasma.
In recent years, vigorous studies are being made on, particularly, method (B) given above, i.e., the CVD method, as a method of forming a gate insulating film. FIGS. 1A to 1D collectively show the CVD method, i.e., method (B). Specifically, FIGS. 1A to 1D are cross sectional views schematically showing the conventional method of forming a silicon nitride film. In the first step, a field oxide film 42 is formed in a thickness of 800 nm by thermal oxidation on a surface of an n-type silicon substrate 41 having a (100) plane as a main surface.
In the next step, pre-treatments with sulfuric acid-hydrogen peroxide (H.sub.2 SO.sub.4 /H.sub.2 O.sub.2), with hydrochloric acid-hydrogen peroxide solution (HCl/H.sub.2 O.sub.2 /H.sub.2 O), and with dilute hydrofluoric acid (HF/H.sub.2 O) are applied successively to the substrate 41 so as to remove organic and metallic contaminants 43 from the surface of the silicon substrate 41. After the pre-treatments, the surface of the substrate 41 is terminated with hydrogen 44 (or modified with hydrogen radical, etc.), as shown in FIG. 1B. Then, the silicon substrate 41 is transferred into a hot wall type reduced pressure CVD furnace, followed by elevating the furnace temperature to 800.degree. C. Under this condition, a mixed gas consisting of SiH.sub.2 Cl.sub.2 and NH.sub.3, which are mixed at a ratio of 1:1, is introduced into the hot wall type reduced pressure CVD furnace so as to deposit a silicon nitride film 45 on the silicon substrate 41, as shown in FIG. 1C. In this step, the total pressure within the CVD furnace is set at 0.8 Torr, and the forming rate of the silicon nitride film 45 is 4 nm/min.
After formation of the silicon nitride film 45, a polycrystalline silicon film 46 is deposited on the silicon nitride film 45 within the CVD furnace using an SiH.sub.4 gas as a raw material, as shown in FIG. 1D, followed by taking the silicon substrate out of the CVD furnace. In this fashion, the silicon nitride film 45 having a thickness of about 6 nm is formed as a gate insulating film.
FIG. 2 schematically shows the construction of the hot wall type reduced pressure CVD furnace used in the method described above. As shown in the drawing, the CVD furnace comprises a reaction tube 51 made of quartz and a silicon substrate supporting disk 52 made of quartz. The SiH.sub.2 Cl.sub.2 gas and the NH.sub.3 gas are supplied into the furnace through a pair of gas supply ports 53. These gases are mixed within the furnace and, then, supplied onto the surface of a silicon substrate 50 held on the disk 52.
It has been clarified recently as a result of vigorous studies that the conventional method outlined above gives rise to serious problems. Specifically, where a silicon nitride film is formed by the CVD method described above, solid particles of ammonium chloride (NH.sub.4 Cl), which is a by-product of the reaction, enter a vacuum pump system 54 so as to make the maintenance and repair of the pump troublesome. Also, it is necessary to remove periodically the silicon nitride film formed on the quartz tube 51 and the substrate supporting disk 52. Further, in the case of forming a silicon nitride film thinner than 6 nm, it is difficult to control the thickness of the silicon nitride film because film-forming rate is high in the CVD method. What should also be noted is that, since the film-forming rate is high, the by-products of NH.sub.4 Cl and H.sub.2 shown in the reaction formula given below tend to enter the silicon nitride film: EQU 3SiH.sub.2 Cl.sub.2 +10NH.sub.3 Si.sub.3 N.sub.4 +6NH.sub.4 Cl+6H.sub.2
Further, since the film-forming temperature in the CVD method is as high as 800.degree. C., traces of oxygen, water, carbide, etc. contained in the atmosphere tend to roughen the surface of the silicon substrate in the step of heating the silicon substrate within the furnace. Still further, the silicon nitride film formed in the initial stage of the film formation is subject to surface diffusion so as to bring about grain growth and, thus, results in failure to obtain a flat film. An additional difficulty to be noted is that, if the substrate temperature is lowered after formation of a thick silicon nitride film, stress is generated at the interface between the silicon substrate and the silicon nitride film because of a difference in thermal expansion coefficient between the two, giving rise to problems such as defects, deformation of the network of the silicon nitride film, and slippage of the silicon substrate.
Serious problems are also generated in the conventional methods other than the CVD method, i.e., method (B). For example, the thermal nitridation of a silicon substrate using an ammonia gas involves a high temperature process exceeding 1,000.degree. C., giving rise to slippage of the silicon substrate. Also, the thickness of the silicon nitride film formed is saturated at about 3 nm. When it comes to the nitridation of the silicon substrate or deposition of a nitride film using plasma, the surface of the silicon substrate is exposed to plasma, with the result that the damage done to the substrate by high energy ions and electrons greatly affects adversely the reliability of the insulating film.
The problems inherent in the conventional film-forming method can be summarized as follows.
First of all, the substrate or the insulating film incurs a damage. For example, the substrate or insulating film incurs a plasma damage in the case of using a parallel plate type radio frequency plasma or an ECR plasma. The electrical characteristics of the product semiconductor device are adversely affected by the plasma damage.
A second problem is that the flatness of the substrate surface is lowered. In the case of forming a very thin nitride film by the CVD method or the thermal nitriding method, the nitride film in the initial film-forming stage is highly sensitive to the surface state of the silicon substrate. As a result, the oxygen and organic materials adsorbed on the substrate surface greatly affect adversely the uniformity of the resultant nitride film, leading to a low flatness of the resultant nitride film. Further, a treatment under a high temperature is required for removing the hydrogen terminating the substrate surface, with the result that the substrate surface is roughened by traces of water, oxygen and organic materials contained in the atmosphere.
A third problem is that it is difficult to control the thickness of the silicon nitride film. In the case of using the CVD method, it is difficult to control the film thickness because the film-forming rate is high in the CVD method. On the other hand, in the case of using a thermal nitriding treatment, it is difficult to form a sufficiently thick nitride film because the thickness of the nitride film is saturated at about 3 nm.
A fourth problem is that the formed nitride film contains a high concentration of oxygen, hydrogen, etc.
When it comes to, for example, a CVD process using SiH.sub.4 and NH.sub.3, a large amount of compounds which are not completely decomposed such as SiH.sub.x and NH.sub.x are taken into the nitride film. Further, the by-product hydrogen, etc. are reported to be recombined with silicon or nitrogen. The hydrogen taken into the nitride film adversely affects the characteristics of the product semiconductor device. For example, the hot carriers of the MOSFET are deteriorated. Also, the resistivity of the silicon substrate is fluctuated. On the other hand, in the case of using an SiF.sub.4 series gas, the hydrogen concentration in the nitride film is lowered. However, a large amount of fluorine is taken into the nitride film, leading to an increase in the trap site.
Further, a fifth problem is that much labor is required for the maintenance and repair of the film-forming apparatus. In the conventional film-forming method, disorder of the vacuum pump tends to be brought about by the side product of the reaction. Also, it is necessary to remove periodically the film deposited on the furnace wall.